There are variations in the characteristics of various circuit elements formed on semiconductor integrated circuits because of variations in manufacturing processes. The variations in the characteristics of the circuit elements cause variations in the characteristics of entire circuits. If the circuit characteristics do not meet specifications, it is necessary to adjust the circuit characteristics by providing an adjuster circuit, for which adjustment resistor trimming is commonly employed.
FIG. 1 is a diagram showing a circuit that adjusts the output voltage Vout of a constant voltage circuit by trimming.
The constant voltage circuit of FIG. 1 includes a reference voltage generator circuit 101 that generates and outputs a predetermined reference voltage Vref, an error amplifier circuit 102, an output transistor M101, and resistors R101 and R102 for output voltage detection. The constant voltage circuit further includes adjusting resistors Rt1 through Rt4 for output voltage adjustment and fuses F101 through F105.
The error amplifier circuit 102 controls the gate voltage of the output transistor M101 so that a divided voltage Vfb generated by dividing the output voltage Vout with the resistors R101, Rt1 through Rt4, and R102 is equal to the reference voltage Vref.
The output voltage Vout before trimming of the fuses F101 through F105 is expressed as Vref×(R101+R102)/R102. In the semiconductor device, the variation of the resistance ratio is small. However, the reference voltage Vref varies within a range of several to tens of % depending on a circuit configuration. Since the output voltage Vout is proportional to the reference voltage Vref, the output voltage Vout also varies.
Therefore, in the constant voltage circuit of FIG. 1, the output voltage Vout before trimming is measured so as to select one of the fuses F101 through F105 which one is not to be cut and is to be left based on the difference between the measured output voltage Vout and a target output voltage, and cut the other fuses. Thereby, it is possible to approximate the output voltage Vout after trimming to the target output voltage. The resistance connected between the resistor R101 and the resistor R102 is the sum of the resistances of the adjusting resistors Rt1 through Rt4, and is constant. irrespective of the trimming condition.
The adjusting resistors Rt1 through Rt4 used in the adjusting circuit shown in FIG. 1 generally have the same resistance, and the number of resistors for output voltage adjustment increases or decreases depending on the range of adjustment and the minimum adjusted value of the output voltage Vout. Therefore, if the range of adjustment is wide and fine adjustment is required, many resistors and fuses are necessary. Therefore, a method for reducing the number of fuses has been developed. (For example, see Japanese Laid-Open Patent Application No. 3-172906.)
FIG. 2 is a circuit diagram showing another conventional constant voltage circuit. In FIG. 2, the same elements as or the elements similar to those of FIG. 1 are referred to by the same reference numerals.
The constant voltage circuit of FIG. 2 is different from that of FIG. 1 in that the fuses F101 through F104 are connected in parallel to the adjusting resistors Rt1 through Rt4. By weighting the resistance of the adjusting resistors Rt1 and Rt2 and the resistance of the adjusting resistors Rt3 and Rt4, it is possible to reduce the number of adjusting resistors and trimming fuses. There is a reference voltage generator circuit using such a method. (For example, see Japanese Patent No. 2639328.)
Further, there is a circuit that reduces the number of adjusting resistors and trimming fuses by disposing a resistor and series circuits each of an adjusting resistor and a trimming fuse in an H shape. (For example, see Japanese Laid-Open Patent Application No. 2001-77310.)
On the other hand, some constant voltage circuits have a variable resistor circuit 112 made in part of output voltage detecting resistors as shown in FIG. 3. (For example, see Japanese Laid-Open Patent Application No. 2004-273103.)
Referring to FIG. 3, the variable resistor circuit 112 includes multiple resistors Rs1 through Rs4 connected in series, switches SW1 through SW4 connected in parallel to the resistors Rs1 through Rs4, respectively, and a selector 113 that performs on-off control on the switches SW1 through SW4 in accordance with an externally input voltage setting signal Sa. In the constant voltage circuit of FIG. 3, the output voltage Vout is changed by varying the resistance of the variable resistor circuit 112. In the constant voltage circuit of FIG. 3, 16 output voltages can be set by performing binary number weighting on the resistors Rs1 through Rs4.
However, the adjusting circuit shown in FIG. 1 has a problem in that a finer unit of adjustment results in an increase in the number of adjusting resistors and fuses. For example, in order to set the range of adjustment with 256-level accuracy, 255 adjusting resistors and 256 fuses are required. The number of fuses can be reduced to eight by employing a method for reducing the number of fuses. However, the number of adjusting resistors cannot be reduced, and in return for a decrease in the number of fuses, as many (256) switching elements as the conventional fuses and a decoder for performing on-off control on each switching element are required, thus causing a problem in that the circuit is extremely large in size.
On the other hand, in the adjusting circuit shown in FIG. 2, the number of resistors and fuses can be significantly reduced by weighting the resistances of the adjusting resistors. In the case of setting the range of adjustment with 256-level accuracy as in the case of FIG. 1, the total number of adjusting resistors and fuses required is 14, that is, seven on each of the upper side and the lower side of the connection part that outputs the divided voltage Vfb. However, this causes a problem in that the combined resistance of the adjusting circuit changes in accordance with the contents of trimming.
Further, in the case of adjusting the output voltage Vout of the constant voltage circuit of FIG. 3 by adding an adjusting circuit whose combined resistance changes in accordance with the contents of trimming as shown in FIG. 2, the current that flows through the resistors for output voltage detection after trimming changes in value in accordance with the trimming result. That is, since the voltage drop in each of the resistors Rs1 through Rs4 for output voltage changing varies, there is a problem in that inputting the same voltage setting signal does not result in the same output voltage Vout depending on the trimming result.
The output voltage Vout may be adjusted by adjusting the reference voltage Vref instead of performing trimming on the resistors for output voltage detection. However, this requires the reference voltage generator circuit itself to be a constant voltage circuit with the same adjusting circuit as shown in FIG. 1 or 2, thus causing an increase in circuit size and in current consumption as well. Further, reducing the number of stages of amplification by an error amplifier circuit in the reference voltage generator circuit in order to suppress an increase in circuit size causes problems such as deterioration of AC characteristics and an increase in ripple.
In the case of using the adjusting circuit shown in FIG. 1 in the constant voltage circuit of FIG. 3, the combined resistance of the resistors for output voltage detection remains the same even after trimming is performed. As described above, however, the adjusting circuit itself is large in circuit size. Accordingly, there is still the problem of an increase in circuit size.
Further, the resistance after trimming also changes in a circuit that reduces the number of adjusting resistors and trimming fuses by disposing a resistor and series circuits each of an adjusting resistor and a trimming fuse in an H shape, so that the same problem as in the case of FIG. 2 occurs. The same problem also occurs in a voltage detector circuit that divides input voltage with resistors and compares the divided voltage with a reference voltage.